Rare cells are often circulating in peripheral blood (1-5). For example, the frequency of circulating trophoblasts obtained from a subject carrying a 47, XYY fetus was approximately one in 10.sup.5 nucleated blood cells after pre-enrichment by gradient fractionation (5). The percentage of mononuclear cells infected with Human Immunodeficiency Virus (HIV) in the peripheral blood of HIV-infected subjects was 0.003-0.01% in asymptomatic subjects and 1-5% in subjects displaying symptoms associated with advanced stages of the disease (6-9) . However, a technique for preserving cells obtained from biological specimens so that rare cells can be analyzed or isolated at a later time has not heretofore been disclosed.
Rare cells are a potential source for early detection or diagnosis of infection, malignancy, or genetic disorder. A technique for preserving cells obtained from biological specimens in a single, unagglutinated state would provide a means for such sensitive diagnosis. A single sample obtained from a subject containing cells which are preserved in a single, unagglutinated state may be analyzed multiple times, varying the parameters of analysis each time as desired yet avoiding the necessity of repeatedly obtaining samples from the same subject. Furthermore, preserved cells from different subjects may be more efficiently analyzed than cells from different subjects maintained in a viable state. Cells from a single subject obtained at different times or stages during the course of a disease may likewise be analyzed in a single, unagglutinated state at one time.